ionization energy

Since Ca has 2 outermost shell eletrons, losing the second electron can help Ca attain the stable noble gas structure, which is favourable.
Yet, for K, it has only one outermost shell electron, losing the second electron will mean to lose an eletron from the K+ ion (which has a stable noble gas structure already), which decrease stability of K, which in term is not favourable.

So second ionization energy of Ca is lower than the second ionization energy of K

After the loss of the first electron of K, the K has a fully filled outermost electronic shell, which has a extra stability. So it is more difficult to loss the second electrons when compare with Ca